Position location systems are well known. In particular, systems for the location of ships based on triangulation of fixed or moving transmitters by the ship are well known. Furthermore, it is known to transmit a signal from a movable object and to find the position of the object by triangulation from fixed receivers or from a single moving receiver.
In the TRANSIT system, six polar transit satellites, whose instantaneous position is well known, transmit a fixed tone. A receiver on a ship receives this signal and determines both the time at which the Doppler shift gradient is a maximum and the magnitude of the Doppler shift. From the time of shift and the known position of the satellite at that time, the latitude is determined. From the magnitude of the shift, the longitude can be determined.
In a variant of the TRANSIT system, called WARK, the ground based object, such as a ship, transmits the signal which is received, with its Doppler shift, to the satellite. The satellite rebroadcasts the signal to a ground station which computes the object's position and transmits it to the object.
Automatic personal locators (APL) and Automatic Vehicle Locators (AVL) are also known in the art. In general these devices are relatively large and transmit substantial power or are small and have a limited range. If the system is small, attempts to increase its range will result in its batteries being consumed too quickly for the system to be practical.
Spread spectrum techniques and, in particular, frequency hopping techniques for communication are also well known. See for example, Digital Communications by, John. G. Proakis, McGraw Hill, New York pp. 845-860. In general these systems have a bandwidth of several kilohertz, which is suitable for information transmission. In view of this requirement, such devices have either a large size or short range or both.
The use of a wide band receiver for the reception of narrow band frequency hopped signals is known in the art. In such systems, signals are received from one or more sources by a wide band receiver and the signals are processed by a fast Fourier transform to provide signals in the frequency domain. A computer divides this spectrum into relatively narrow preset communication channels, which correspond to the channels which are used by the transmitting stations. When a plurality of transmitting stations are used, the system is capable of separating between them and can thus receive more than one message, although the problem of de-interleaving of the signals becomes difficult when large numbers of potential or actual transmitters are involved.
Location systems for vehicles based on GPS or Loran-C are also known. In these systems a mobile receiver receives the GPS or Loran-C signals, determines its own position and broadcasts this position to a central station.